Crystal element for piezo sensors

ABSTRACT

The invention relates to a piezoelectric crystal with transversal effect which may be fitted into a sensor without centering effort or the use of other means, yet assuring a centered and aligned position. For this the crystal according to the invention has a base with at least individual circular segments, which can be fitted into a drill provided for it in a holding device of the sensor. Moreover the crystal production can be automated for mass production, thereby lowering its costs.

[0001] The invention relates to a piezoelectric crystal with transversal effect.

[0002] Piezoelectric crystals are employed in various sensors for measuring forces, pressures, accelerations, strains and moments. For this purpose, crystals with transversal effect are cut into thin plates or rods for example. For metrological uses these thin plates are exposed typically to a pressure on the small end surfaces of the plate, causing an electrical charge to appear on the two large side surfaces. By placing an electrically conductive layer on the two side surfaces, which however have no electrical contact with each other, this charge is measured with the appropriate device in the sensor so that information about the pressure is obtained and may be transmitted further. Such sensors are familiar.

[0003] What is crucial, however, is that the crystal is fitted vertically and centered in the axis of the sensor. Any slight tilt will result in a false measurement or fracture of the crystal under the influence of the forces occurring subsequently. A contact of the crystal to the edge of the sensor may lead to a short circuit or hysteresis.

[0004] Since the sensitivity of the crystal is proportional to the ratio of the charge pickup surface to the pressure surface, these conventional crystal plates are very thin. Hence the handling, especially the centering and aligning in the sensor, are very difficult and laborious.

[0005] Often the sensor is fitted with centering aids which hold the crystal in position. However the various materials of these centering aids do not tolerate very high temperatures. Consequently the application area of the sensor as a whole is limited to a lower maximum temperature.

[0006] The present invention is intended to provide a piezoelectric crystal which can be fitted easily into the sensor without laborious centering and aligning, and without restriction to a lower temperature range. Furthermore a process is indicated that enables the crystal to be manufactured rationally and in large quantities, at low cost and fully automatically.

[0007] This task is solved according to the invention by a crystal as described in claim 1, an arrangement of crystals according to claim 6, a sensor according to claim 7 and a process according to claim 8.

[0008] The invention is explained by reference to the following drawings:

[0009]FIG. 1 Piezoelectric crystal fitted in a sensor embodying the state of the art.

[0010]FIG. 2 Piezoelectric crystal arrangement fitted in a sensor embodying the state of the art.

[0011]FIG. 3 Embodiment of a piezoelectric crystal according to the invention.

[0012]FIG. 4 A crystal according to the invention in fitted position.

[0013]FIG. 5 Wafer crystals according to the invention in production.

[0014]FIG. 6 Another possible embodiment of a crystal according to the invention.

[0015]FIG. 7 Another possible embodiment of a crystal according to the invention and an arrangement of such a crystal according to the invention.

[0016]FIG. 1a shows a side view of a piezoelectric crystal (11) with transversal effect in the form of a plate, fitted in a sensor (10), as is familiar from the state of the art. The crystal (11) is clamped at its ends by holding devices (12, 13), to retain it in the required position. FIG. 1b shows the cross section of a crystal (11) fitted in a sensor (10) in this way in a sleeve (14). The charge (+, −) is led off from the two side surfaces (15, 16) on electrically conductive layers provided for this purpose. To achieve this one electrically conductive layer (17) leads to an upper holding device (12), while the other layer (18) leads to a lower holding device on the opposite side. Accordingly the two holding devices (17, 18) have opposite electrical charges.

[0017]FIG. 2 shows another embodiment of the state of the art. In this embodiment typically three identical crystal rods 11, their cross sections having the form of circle segments, are disposed in a circle as in FIG. 2. The charge on the outer surfaces of the crystals 11 is picked up via a sleeve 14 or through one end for example. The other pole on the inside of the crystals is picked up via an electrically conducting spiral 19, as shown in FIG. 2a. The spiral 19 also acts as a centering aid for the crystals 11, which in turn are held from outside on a sleeve 14.

[0018]FIG. 3 shows a crystal 20 with transversal effect according to the invention. This crystal consists preferentially of a monocrystalline material, whose symmetry of the piezoelectric d tensor corresponds to that of the point group 32. This crystal 29 is distinguished by a base 21 which has a plate 22 attached at one end. This base 21 projects beyond the thickness of the plate 22 on one side at least. According to the invention the transition from the end of the plate 24 to the base 21 may have a curvature 23 to enhance the stability of the base and plate system, though other transitions are possible too. At the end of the plate 24 opposite the base 21 and on the bottom of the base 25, bevels 26 may be provided to prevent the edges breaking out.

[0019] The two long side surfaces of the plate 15, 16 are each coated with an electrically conductive layer 17, 18, making a charge transport possible. One layer 17 runs on to the top end of the crystal plate. The layer 18 on the opposite side runs on over the edge of the base 28 to the bottom of the base 25. If the crystal 20 is clamped by suitable holding devices 12, 13, opposed charges can be picked up on the bottom of the base 25 and on the end of the plate 24. Here it is essential that the electrically conductive layers 17, 18 are insulated electrically from each other. To ensure this an insulating bevel 29 is provided at the top edge of the crystal plate (24), at which the electrically conductive layers 17, 18 are interrupted.

[0020]FIG. 4a shows the crystal 20 according to the invention as fitted in a sensor 10. The bottom holding device 13 has a drill 30 into which the base 21 can be fitted. This drill 30 must be less deep than the edge of the base 28, to ensure that the side of the electrically conductive layer 17 led upwards has no electrical contact with the edge of the drill (30). The other side of the electrically conductive layer 18 must have a good electrical contact at the bottom of the drill 30.

[0021]FIG. 4b shows a cross section of the crystal 20 according to the invention fitted into the base 21. It will be seen that the edge of the base 28 has partially rounded contours 31. This ensures that the crystal 20 fits into the drill 30 of the holding device 13. The curvature 31 may be continuous in the side of the crystal and extend over the end faces 32 of the crystal plate. The curvature 31 on the base 21 should not be continuous on the side 33 parallel to the crystal plate, otherwise this curvature 31 would remove the electrically conductive layer 18 completely, and this would have to be restored again to assure contact with the bottom surface of the base. By forgoing a complete curvature 31 in this part, the electrically conductive layer 18 is retained and contact with the electrically conductive layer of the bottom of the base 25 is assured.

[0022] The process for the mass production of such crystals according to the invention is illustrated in FIG. 5. A crystal wafer 40, in rectangular form for example, is cut in a first process stage so that a plate 41 of the desired thickness is obtained with a base ledge 42 running along one edge of the plate 41. Here it is essential that the transition from the plate 41 to the base ledge 42 has a curvature 23 in accordance with the invention. In a further process stage the crystal wafer 40 is coated completely with an electrically conductive layer except for the end faces 43. After this the electrically conductive layer is broken through at two areas. One of these areas is the edge of the base ledge 44 on one side. The other area is preferentially along the top edge and on the other side of the crystal wafer 40. At this area it is advisable to provide an insulating phase 45. This results in two electrically conductive layers 16, 17 isolated electrically from each other.

[0023] In a further process step the crystal wafer 40 is divided into two or more smaller crystals 20, all having a base 21 and already the electrically conductive layers 16, 17. On each base 21 of these crystals 20 is at least one curvature (31), which may be provided on only one of the four corners of the base 21 or extend over the sides of the plate 22 without interruption.

[0024] In a subsequent process step each crystal 20 may be provided with further bevels 26 on the bottom edge of the base and along the edge of the end face 24, whereby the electrically conductive layer 17 on the end face 24 must not be interrupted. A bevel (26) must be produced on the crystal wafer 40 at this point before the electrically conductive layer is applied to the wafer 40.

[0025] When inserting the crystal 20 it must enter the drill 30 of the holding device 13 with the side of the base 21. Care must be taken to ensure that the drill 30 is large enough so that the base 21 has play in it. The edge of the base 28 must be about twice as high as the depth of the drill 30. The crystal 20 is not clamped in the drill 30, but is held sufficiently to allow the second holding device 12 to be fitted on the opposite side without the crystal 20 being able to shift off-centre or tilt.

[0026] The overall height of such a crystal 20 according to the invention must lie between 1 and 40 mm, preferably between 2 and 10 mm. According to the invention the height of the base, including the rounded contours 31 to the crystal plate, is {fraction (1/10)}^(th) to ⅓^(rd) of the overall height of the crystal 20. The crystals described here are suited for use in metrology, in particular for measuring forces, pressures, accelerations, moments and strains.

[0027] Another configuration of the crystal 20 is shown in FIG. 6. This double crystal 50 consists of two crystal plates 22 joined by a common base 21. The advantage of this arrangement lies in the double load capacity of the crystal 50 under pressure or force, with the same sensitivity and overall height. Other configurations with more than two plates 22 are also possible. With these or similar configurations it must be ensured that each crystal plate 22 has an electrically conductive layer 17, 18 on both sides, with the two layers 17, 18 of a plate 22 having different holding devices 12, 13 in electrically conductive contact and insulated from other, differently charged electrically conductive layers 18, 17. For this the necessary insulating surfaces and bevels 29 must be applied. With arrangements having several plates, it may be advantageous to provide side notches or holes whose surface is provided with electrically conductive layers 18 in order to conduct the charge to the bottom of the base 25.

[0028] Another embodiment of the crystal is shown in FIG. 7. This embodiment consists of two identical crystals 29, each having a base projecting beyond the crystal plate 22 at one side only. The sides of the crystal 15 having no base can thus be placed together. Since they have the same polarity, short-circuiting upon contact is ruled out. The production of this crystal is analogous to the production of crystals already described.

[0029] Placing two such crystals together gives a configuration very similar to that of one crystal in FIG. 3, though with the difference that with equal geometrical conditions the same force may be applied onto the crystal or crystals, but the sensitivity of the configuration with the double crystal 50 is twice as high, because the surface of the charge pickups in FIG. 7 is double. With the doubled width of the crystal plates 22, the load capacity is doubled for the same sensitivity. With regard to self-centering this typical arrangement of the double crystal 50 in FIG. 7 is equal to the single crystal in FIG. 3.

[0030] In all these examples there is no need for ancillary materials to facilitate centering. Consequently the application range of the sensor is subject to no restrictions due to temperature ranges.

List of Designations

[0031]10 Sensor (state of the art)

[0032]11 Crystal

[0033]12 Holding device (top)

[0034]13 Holding device (bottom)

[0035]14 Sleeve

[0036]15 Side of crystal

[0037]16 Side of crystal

[0038]17 Electrically conductive layer (e.g. −)

[0039]18 Electrically conductive layer (e.g. +)

[0040]19 Electrically conductive spiral

[0041]20 Crystal according to the invention

[0042]21 Base

[0043]22 Plate

[0044]23 Curvature

[0045]24 End face

[0046]25 Bottom of base

[0047]26 Bevel

[0048]28 Edge of base

[0049]29 Insulating bevel

[0050]30 Drill

[0051]31 Rounded contours

[0052]32 Side faces of crystal plate

[0053]33 Side of the base parallel with the crystal plate

[0054]40 Crystal wafer

[0055]41 Plate

[0056]42 Base ledge

[0057]43 End faces

[0058]44 Edge of base ledge

[0059]45 Insulating bevel

[0060]50 Double crystal 

1. Piezoelectric crystal (20) with transversal effect, characterized by the crystal (20) having at least one flat plate (22) and at least one base (21) at right angles to this plate, the base (21) projecting beyond the thickness of the plate (22) on one side at least.
 2. Crystal according to claim 1, characterized by at least both surfaces of the plate (15, 16) having an electrically conductive layer (17, 18).
 3. Crystal according to claim 2, characterized by having at least two electrically conductive layers (17, 18) on it, insulated from each other electrically.
 4. Crystal according to claim 3, characterized by one of the layers (18) extending as far as the bottom of the base (25), and another (17) extending to the end face (24) of the plate.
 5. Crystal according to claim 1, characterized by the edge of the base (28) being circular at least in part.
 6. Arrangement of two identical crystals according to one of claims 1 to 5, characterized by the base (21) projecting exactly one-sided over the plate (22) and the two crystals being placed together at the sides without bases (15), having the same polarity.
 7. Piezoelectric sensor (10) for detecting force, pressure or acceleration signals by the means of at least one piezoelectric crystal (20) with transversal effect inside it, characterized by the the crystal (20) having the form of at least one flat plate (22) and at least one base (21) at right angles to it, the base (21) projecting beyond the thickness of the plate (22) on one side at least.
 8. Process for producing piezoelectric crystals (20) with transversal effect, characterized by the shaping of the crystal wafer (40) to a desired plate thickness (41) leaving a base ledge (42), by applying a coating of electrically conductive material to the surface, by providing at least two insulating bevels (45) to divide the electrically conductive layer into two layers (17, 18) separated electrically from each other, and by the division of the crystal wafer (40) into two or more smaller plates (22) with bases (21).
 9. Process according to claim 8, characterized by an insulating bevel (29) provided on one side of the base ledge (42) and another insulating bevel (29) diametrical to it on the edge of the crystal wafer (40).
 10. Process according to claim 8 or 9, characterized by the bases (21) of the resulting crystal being provided partially at least with rounded contours (31).
 11. Process according to claim 8 or 9, characterized by the edges on the end face of the crystal plate (24) and/or at the bottom of the base (25) having bevels (26). 